Stack two identical full-range loudspeakers as shown in Fig. 4. Carefully align the HF horns
and wire the speakers in mono. Stand in front while listening to your favorite full-spectrum
CD. Ask a friend to move the top speaker slowly away from you. The degradation in sound
quality you hear is caused by comb filters. The experiment is most dramatic when you use
good quality speakers.
Correcting Comb Filters
Comb filters are inevitable to some degree in every live sound system, and they cannot be
corrected with equalization. Fortunately, most comb filter problems can be reduced to a
minimum by synchronizing the signals and reducing the amplitude of the delayed signal. The
examples below show several practical applications.
The Precedence Effect: Aligning the Acoustic Image
Helmut Haas published a study in 1951 describing a series of experiments that demonstrates
how people perceive delayed signals and echoes. In his experiments, a listener was posi-
tioned between two speakers placed 3 meters away; one was placed 45 degrees to the right
and the other was placed 45 degrees to the left. When the same program was played
through both speakers simultaneously, the listener perceived the acoustic image (the
direction from which the sound seemed to be coming) centered between the speakers.
When Haas delayed the signal going to one of the speakers by somewhere between 5 to 35
milliseconds, the listener perceived a shift in the acoustic image to the speaker heard first.
While the delayed speaker did not contribute to the apparent direction of the sound, it did
make the program seem louder and “fuller.”
Haas showed that you must increase the loudness of the delayed signal by about 8 to 10 dB
(twice the perceived loudness) in order for the acoustic image to move back to the original
center position. Increasing the loudness more than this, or increasing the delay somewhat
more than 35 milliseconds, makes the delayed signal sound like an echo.
The phenomenon describing how the acoustic image follows the signal we hear first is called
the Precedence Effect. The phenomenon that makes two distinct sounds heard less than 35
msec. apart seem like only one sound is call the Haas Effect. However, the terms are often
used interchangeably in the sound industry.
THREE APPLICATIONS
APPLICATION I: Under-The-Balcony Speakers
Fig. 5 above shows a typical situation where the performer is amplified by a center cluster
hanging above the stage. Almost everybody in the audience will enjoy good sound, except
those seated in the shadow of the balcony. So we add an under-balcony speaker to fill in the
shadow.
6
Fig. 5:
Overhead view
of under-balcony
application.
